1. Field of the Invention
The present invention relates to micromachined mirrors for use in optical switching, steering and scanning systems and, more particularly, to micromachined mirrors for use in optical data tracking, storage and retrieval systems.
2. Background
Electrostatic pull-in is a phenomenon that limits the range of electrostatically driven deflectable micromachined devices. In general, pull-in occurs when the nonlinear electrostatic drive overwhelms the capabilities of the device""s mechanical suspension to achieve equilibrium with the electrostatic forces. In a torsional mirror, such as of the type described in copending U.S. patent application Ser. No. 09/231,317 filed Jan. 13, 1999, the electrostatic drive causes rotation of the mirror plate about the axis defined by the torsional hinge suspension. An equilibrium angular deflection is achieved when the restoring torque provided by the two torsional hinges balances the electrostatic attraction torque provided by the drive electrode. The torsional hinge suspension provides a restoring torque that is proportional to the angle of rotation of the mirror plate. However, the electrostatic torque increases nonlinearly as the separation between the drive electrode and the grounded mirror plate is decreased by the rotation of the mirror plate. At some value of angular deflection, the electrostatic torque becomes larger than what can be balanced by the linear restoring torque of the hinges. At this pull-in angle, the outer edge of the mirror plate spontaneously deflects across the remainder of the electrostatic gap thus limiting the useful angular range of the mirror to less than that which results in pull-in.
The issue of electrostatic pull-in has been presented and analyzed in several publications. For example, Seeger and Crary, Stabilization of Electrostatically Actuated Mechanical Devices, Proc. Transducers ""97, Chicago, Ill., pp. 1133-1136, June 1997, present an approach towards preventing the pull-in phenomenon from occurring in translational electrostatic actuators. Their method places a feedback capacitor in series with the device which essentially modifies the potential energy function of the system to the point where no unstable operating points exist as the movable plate is driven towards the drive electrode. Although this method can be used to increase the stable range of torsional electrostatic devices, it has the undesirable tradeoff that the actuation voltage has to be dramatically increased in order to charge the feedback capacitor.
The issue of electrostatic pull-in for translational micromachined mirrors was discussed in Burns and Bright, Nonlinear Flexures for Stable Deflection of an Electrostatically Actuated Micromirror, Proc. SPIE, Vol. 3226, Austin, Tex. Sep. 1997. In this paper, a theoretical argument for the use of flexures with nonlinear deflection performance is presented. However, a design that provides for nonlinear performance is not presented. Rather, a design for a composite flexure comprising a primary and an auxiliary wherein beyond a certain deflection of the primary flexure the restoring force provided by the auxiliary flexure is combined additively to that of the primary flexure is disclosed. As such, the flexural design is piecewise linear rather than truly nonlinear.
The suspension means for both translational and torsional electrostatically actuated micromachined devices are typically modeled as slender beams or thin diaphragms. For small deflections and rotations, these structures behave linearly. Thus, the load acting on the structure is proportional to its deflection with the constant of proportionality equal to the spring constant specific to that direction of deformation. For large deflections or rotations, these structures no longer respond linearly to applied loads. The form of the non-linearity is highly dependent on the specifics of the suspension geometry but generally takes on a relationship that is approximately a superposition of the small deflection linear term together with a cubic term which dominates for the larger deflections and rotations. In Jerman, The Fabrication and Use of Micromachined Corrugated Silicon Diaphragms, Sensors and Actuators, A21-A23 (1990) pp. 988-992 and U.S. Pat. No. 5,116,457 for Semiconductor Transducer or Actuator Utilizing Corrugated Supports to Jerman, an example of such a relationship is given where thin diaphragms are used to support the central boss of a micromachined structure. However, in these references Jerman does not make productive use of the non-linear deflection characteristics of these diaphragms.
What is needed is an improved micromachined mirror assembly having a restoring torque that increases nonlinearly with the deflection angle of the mirror to compensate for the nonlinear electrostatic drive forces of the mirror assembly. Such a mirror assembly would preferably increase the pull-in angle so as to increase the effective deflection range of the mirror.
The invention includes a mirror assembly of micron dimensions for use in deflecting a beam of light. The mirror assembly includes a planar base and a planar mirror spaced apart from the planar base and disposed generally parallel to the planar base. The planar mirror has first and second end portions and a longitudinal axis extending between the first and second end portions. First and second torsional members extend along the longitudinal axis and are connected to the respective first and second end portions for permitting the mirror to rock between first and second positions about the longitudinal axis relative to the planar base. Means is included for securing the first and second torsional members to the planar base. At least a portion of the mirror is made from a conductive material. First and second spaced-apart electrodes are carried by the planar base for driving the mirror between the first and second positions. A tether member extends transversely of the longitudinal axis and is secured to the first torsional member. Means is provided for securing the tether member to the planar base. The tether member regulates the rocking of the mirror.